1. Technical Field
The present invention relates to an image forming apparatus and an image forming method of developing an electrostatic latent image with toner while a latent image holder for holding an electrostatic latent image and a toner carrying roller for carrying toner are opposed to each other without being in contact.
2. Related Art
As a technique for developing an electrostatic latent image with toner, a technique of disposing a latent image holder for holding an electrostatic latent image and a toner carrying roller for carrying toner to be opposed to each other with a gap therebetween, and then developing an electrostatic latent image by causing the toner to fly into the gap, that is, a so-called non-contact developing method is known (for example, refer to JP-A-2007-127800). For this type of image forming apparatus, toner with a volume average particle size of 8 to 10 μm has been widely used. However, for the purpose of implementing high definition of images, speeding up processes, and decreasing fixing temperature, an additional reduction in particle size (for example, to cause the volume average particle size to be 5 μm or less) of toner is necessary.
However, it has been recently discovered that toner with such a smaller particle size exhibits different behavior from that of toner with a large diameter. For example, image force or van der Waals force exerted on charged toner with a small size by the toner carrying roller is increased, and it becomes difficult for the charged toner to fly from the toner carrying roller. Accordingly, it becomes difficult to develop an image with sufficient density. In addition, toner with a small particle size and a small mass has a property that it scatters easily and adheres to the inside or the outside of the image forming apparatus as a base fog. This results in the image being dirty.
Here, by increasing an amount of toner transported on the toner carrying roller or strengthening an electric field generated in the gap between the latent image holder and the toner carrying roller, it is possible to compensate for the insufficiency of the developing density. However, in this case, a larger amount of toner scatters, or a discharge occurs between the latent image holder and the toner carrying roller, resulting in an image being muddy. As described above, the object of obtaining a sufficient developing density and the object of suppressing toner scattering to the inside or the outside of an apparatus and suppressing base fog conflict with each other. In order to reconcile both these objects and achieve a reduction in toner particle size, the existing technique has to be improved.